undescended testis vs undescended testicle

undescended testis and undescended testicle both are nouns.

Word	Noun	Adjective	Verb	Adverb
undescended testis	Yes	No	No	No
undescended testicle	Yes	No	No	No

As nouns, undescended testis and undescended testicle are synonyms defined as:

undescended testis and undescended testicle: a testis that fails to move into the scrotum as the male fetus develops

undescended testis (noun)	undescended testicle (noun)
a testis that fails to move into the scrotum as the male fetus develops	a testis that fails to move into the scrotum as the male fetus develops